Mosquito Systematics vol. lO(3) 1978 301

Comparative Structure of the Labiohypopharynx of Fourth Stage Mosquito Larvae (Diptera: Culicidae), with Comments on Larval Morphology, Evolution and Feeding Habits*

Ralph E. Harbach Department of Entomology North Carolina State University Raleigh, North Carolina 27650

ABSTRACT. The labiohypopharynges of 62 species of fourth stage culicid larvae representing 3 genera of the Dixinae, 2 of the Chaoborinae and all but one (Fica%ia) of the 34 genera recognized by Knight and Stone (1977) have been studied with the light and scanning electron microscopes. The structure of the labiohypopharynx is described and illustrated for at least one species of each genus examined. Phylogenetic relationships are discussed in regard to larval morphology and feeding behavior.

INTRODUCTION

This paper concludes a series of comparative studies on the mouthparts of fourth stage culicid larvae. Knight (1971) examined the anatomy of the mandi- bles, determined the homologies of the mandibular substructures and developed a structural terminology "suitable for use in taxonomic studies." A similar study was conducted on the maxillae by Knight and Harbach (1977).

The comparative examination of the labiohypopharynx made by Harbach and Knight (1977b) paved the way for the present paper. Although the structural interpretation presented herein is basically the same as that given by these authors, a new terminology has been devised which reflects a more refined in- terpretation of structure and conforms more closely to that applied to other insects. The references cited by Harbach and Knight should be consulted for a comprehensive introduction to the literature on the morphology of the labio- hypopharynx.

Although Montschadsky (1936), Surtees (1959) and Harbach (1977) have out- lined certain evolutionary trends based on correlations between mouthpart struc- ture and feeding habit, the evolution of culicid larvae is still a much vexed question. In order to shed additional light on larval phylogeny, evolutionary relationships which I have derived from the aforementioned series of studies, as well as observations made on other aspects of larval morphology, are set forth in the discussion.

MATERIALS AND METHODS

Light and scanning electron microscope investigations were conducted on the labiohypopharynges of alcohol-preserved fourth stage larvae. Material for *This publication was supported in part by NIH Grant LM02787 to Kenneth L. Knight from the National Library of Medicine. 302

examination under the light microscope was prepared and mounted on glass slides according to the procedures of Wirth (1961) and Knight (1971). For the scanning electron microscope, heads were Freon critical-point-dried and mounted anterior end up on aluminum stubs. Once secured, the median labral plate, palatum, an- tennae, mandibles and maxillae were carefully removed from each head in order that the topography of the labiohypopharynx could be examined without obstruc- tion. The labiohypopharynges of one to several larvae were prepared for both the light and scanning electron microscopes.

Concerning the figures, each drawing depicts an anterior or near anterior aspect of a labiohypopharynx with its dorsal extremity uppermost. All scale lines are equal to 0.05 mm. The species are grouped into subfamilies and tribes, otherwise the linear order of the taxa has no phylogenetic import. Species stu- died in addition to those illustrated are listed in an appendix.

Following Edwards (1932), the family Culicidae is considered to include the Dixinae and Chaoborinae. The taxa constituting Edwards' subfamily Culicinae, however, are recognized here as belonging to the subfamilies Anophelinae, Culi- cinae and Toxorhynchitinae of Knight and Stone (1977). The tribes are those of Belkin (1962).

OBSERVATIONS

The labium of generalized insects is divided into a proximal postlabium comprising the submentum and mentum and a distal prelabium comprising the pre- mentum, glossae, paraglossae and the labial palpi. In most nematocerous larvae the postlabium has been largely incorporated into the ventral surface of the head capsule (nematocerous larvae are basically prognathous) with only the dis- tal portion of the mentum projecting beyond the anterior margin as a dentate lobe or ridge. The prelabium has united with the hypopharynx to form the compo- site tabiohypopharynx (LbHy). In culicid larvae the labiohypopharynx is attach- ed to the head between the mouth and the dorsomerrtwn (Dm), a transverse subdivi- sion of the mentum found in both culicid (Laffoon and Knight 1973) and chironomid larvae (Saether 1971), where it forms the protruding posterior wall of the pre- oral cavity (Figs. 1,2). The component parts of the labiohypopharynx, the dor- sal hypopharynx (Hy) and the ventral pretabiwn, are joined and supported dorsally by the cibaria2 bars (CB) extending from lateral areas of the cranium. Ventrally the labiohypopharynx is reflected into the mentum at the base of the dorsomentum (Figs. 1,2).

Hypopharynx

The fleshy hypopharyngeal lobe, often strengthened internally by a sclero- tized hypopharyngea2bar (HUB) (see e.g., Fig. 16), is separated from the scle- rotized prelabium by a slitlike saZ

In most Culicinae the hypopharynx bears a pair of earlike lobes, the super- ~inquae (Sl) (see e.g., Figs. 9,13,28,31). These are often imperceptible when observed with the light microscope owing to their membranous nature. As in generalized insects, the superlinguae have united with the median Z~?Z@U (L) to form the definitive hypopharynx.

The adoral surface of the hypopharynx of Zeugnomy&z bears a row of branched cuticular processes (Fig. 23) not unlike the "dichotomous bristles' which occur on the hypopharynx of certain simuliid larvae (Craig 1977). The labiohypopharynx of dixine larvae is unique in that the lateral sides of the hypopharyngeal com- ponent support a pair of denticulate tritural surfaces, the ~~popharyngeai!Mae (HyM) (Figs. 3,4).

Prelabium

The prelabial component of the labiohypopharynx is comprised chiefly of the labial prementtmt, and it is probable that a pair of extrinsic muscles which originate on the ventral cranial wall and insert laterally on often indisting- uishable prementat apodemes(PAP) (see e.g., Figs. 5,14,24,34) is one of two pairs of tentorio-premental muscles found in generalized insects. The central region of the prementum is commonly elevated and bears a variously developed collection of usually strongly sclerotized toothed or toothlike projections which encompass a pair of oval membranous areas. As has been suggested by other authors (Christophers 1960; Chaudonneret 1962), the membranous areas represent vestigial ZabiaZ patpi (LP). Separating the palpi in the Anophelinae and cer- tain Culicinae is a cusped tonguelike structure which seemingly corresponds to the fused glossae and paraglossae, the ZiquZa (Lg), of many other insects,

LiguZa. The anopheline ligula (Figs. 5-7) strongly resembles the "ligula" (Saether 1971) of certain chironomid larvae (see the figures of Johannsen 1937). In many culicine larvae, notably those of the genera CuZex (Fig. 11) and Aedes (Fig. 27), the ligula is a recumbent troughlike structure which bears resembl- ance to the "ligula" of simuliid larvae (Craig 1977). In Paradkca, toxorhyn- chitine, and some culicine larvae, including most Sabethini, the ligula is un- recognizably fused with some of the toothlike elements of the prementum (see below).

Labia2 paLpi. Each labial palpus commonly bears four sensory peglike structures (yin 1970), Labial pazpaz sensoria 1-4 (LPS , LPS2, LPS , LPS4), which are struc- turally similar to chemoreceptors known to &ave a gusta 2 ory function in other insects. Sensoria 1 and 2 often arise from a commonpedicel (see e.g., Figs. 25,37).

Located on each side of the ligula in anopheline larvae is a scale- or leaflike process. These processes correspond exactly to the "paraglossae" (Saether 1971) of certain chironomid larvae. Das (1937), however, maintains that the paraglossae and glossae "generally form a single lobe" in larval in- sects. Since the structures arise from the labial palpi and appear to have an ill-defined socket, they are considered here as Zabiat pazpaz sensoria 5 (LPS5) (Figs. 5,6). 304

Prementum. In general, the most conspicuous feature of the culicid prementum is the intricate assemblage of toothlike processes which subtend the dorsal and lateral margins of the labial palpi. In the generalized condition exemplified by Opifea: fuscus (Fig. 21), these processes can be conveniently subdivided into three groups: 1) the bilaterally paired prementat teeth comprising (a) an inter- connected group of ZateraZ pwmentaZ teeth (LPT) which flank the labial palpi, and (b) a prementaz denta arch @DA) which curves around the dorsal margin of each palpus and is united with the base of the ligula; 2) a cluster of premental CUSPS (PCu) located centrally between the premental dental arches; and 3) a patch of numerous peglike processes, a prementai! maZa (PM), borne on each slop- ing dorsolateral surface adjacent to the lateral premental teeth.

The premental teeth are weakly developed in dixine and anopheline larvae where they are typically represented by a pair of heavily sclerotized cusped ridges which bend laterally around the dorsal margins of the labial palpi. The premental teeth of culicine larvae are normally well developed but vary widely in specific form. Except in most sabethines, the lateral premental teeth of culicine larvae generally consist of three interconnected groups which from dor- sal to ventral are designated titera prementaz teeth I-3(LPTl, LPT2, LPT3). These are quire distinct in HOdge8ti (Fig. 15) and Mansoniini larvae (Figs. 18, 19), but in certain other genera LPTl are weakly developed and difficult to distinguish. In these cases the LPT of each side appear to be reduced and co- alesced with the lateral portion of t he premental dental arch (see e.g., Figs. 11,12,16,24). In toxorhynchitine (Figs. 39,40) and most sabethine species (Figs. 32,34038), the lateral premental teeth of each side consist of a crescentic mass of dentate or dentate-serrate projections.

The trend in culicines is for the premental dental arches to separate from the lateral premental teeth and to fuse with the premental cusps and the ligula (see e.g., Fig. 26). In the more complex state exhibited by many sabethine species, these structures are unrecognizably fused with the latter to form the toothed triangular to T-shaped mass of prementoZigu&zr teeth (PLT) (Figs. 30-38). The ridge of prementoligular teeth which separates the labial palpi of Paraditi (Fig. 4) probably consists largely of a highly modified ligula.

In anopheline larvae, an elevated median longitudinal row of prementaz ridge teeth (PRT) extends dorsally from the premental teeth and separates a pair of broad, slanting, denticulate tritural surfaces (Figs. 5-7). These are seemingly homologous for the most part with the culicine premental cusps. The expansive tritural surfaces are largely homologous with the premental malae of @if&r although some of the ventromedial denticles of each may be homologous with a portion of the premental cusps.

The culicine premental cusps are evidently homologous with a somewhat larger group of similar structures borne centrally on the labiohypopharynx of dixine larvae (Figs. 3,4). The anopheline premental malae correspond function- ally to the hypopharyngeal malae of dixines.

In most culicines, the area lying between the salivary meatus and the premental teeth is somewhat membranous and is believed to correspond to the Mosquito Systematics Vol. lO(3) 1978 305

adoral wall of the generalized insect prementum. This area commonly bears a pair of prementat sensoria (PS) (see e.g., Figs.19,29) which are innervated by branches of the labial nerve and have the structure of typical insect chemo- receptors (Yin 1970). The premental sensoria of anopheline larvae (Figs. 5-7) are quite small and sometimes difficult to distinguish from the denticles of the premental malae which border them. They give the impression of having been displaced dorsally by the well developed premental malae.

On the sloping lateral surfaces of the prementum immediately adjacent to each aggregate of lateral premental teeth is a group of ZateraZ prementat pro- cesses (LPP) . These occur principally in aedine species (Figs. 21,22,24,27) but are also found in species of CztZex (Fig. ll), W?iseta (Fig. 16) and O&ho-- podomysa(Fig. 20). The dixine homologues (Figs. 2,3) are located at the late- ral margins of the labial palpi (these were incorrectly considered as lateral prelabial [= premental] teeth by Harbach and Knight 1977b). Equivalent struc- tures in Eucorethra underuoodi (Fig. 41) are located dorsolateral to the cluster of labial palpal sensoria.

A group of ventral prementaz processes (VPP) arises near the ventral mar- gin of the labiohypopharynx. In aedine (Figs. 21-25,27,28) and Cdiseta spe- cies (Fig. 16) these are generally numerous and scalelike whereas in most other culicines they are usually few in number and spine- or lobelike. The processes are weakly developed in species of AnopheZes (Fig. 6) and Chugasia (Fig. 7). They are absent in both the BironeZZa (Fig. 5) and Toxorhyn&{tes species (Figs. 39,40) examined. The corresponding structures of Eucorethra (Fig. 41) and dixine larvae (Figs. 2,3) resemble those of certain aedine species.

A peculiar heart-shaped flap, the prementaZ cordate process (PCP), arises immediately ventral to the ligula in anophelines (Figs. 5-7). The structure dorsally opposes the ventral premental processes of AnopheZes (Fig. 6) and Chagasia (Fig. 7). In BironeZZa (Fig. 5) it has a distinct bilobed form and is similar in appearance to the ventral premental processes of lkznotaenia (Fig. 8), Aedeomyia (Fig. 14), Hodgesia (Fig. 15) and Mansoniini species (Figs. 18,19) with which it could be homologous. 306

LIST OF FIGURES

Fig. 1. Aedes (OchZerotatusl fulvus patlens Ross. Anterior aspect of head showing position of labiohypopharynx. Eyes, antennae, palatal brushes, and articulating membranes omitted; mandibles and maxillae shown in cross-section. (Redrawn from Laffoon and Knight 1973)

Fig. 2. AnopheZesIAnophelesl qzuzirimzcutatus Say. Same as Fig. 1 except mandibles and maxillary palpi intact with maxillary bodies omitted. (Redrawn from Laffoon and Knight 1973)

The labiohypopharynges of

Fig. 3. Nothodixa campbeZi (Alexander). Fig. 4. Paradixa neozelandica (Tonnoir). (Redrawn from Harbach and Knight 1977b) Fig. 5. BironeZZa (BrugeZZa) hoZZandi Taylor.

Fig. 6. AnopheZes (Nyssorhynchus) atbtianus Wiedemannl (Redrawn from Harbach and Knight 1977b) Fig. 7. ChagasCa bathana (Dyar). Fig. 8. Vranotaenia (Vranotaenia) barnesi Belkin. Fig. 9. Vranotaenia (PseudoficaZbCa) Zagunensis Baisas. Fig. 10. CuZex (Lutz&) haZifaxii Theobald. Fig. 11. CuZex (CuZez) nigripazpus Theobald. Fig. 12. GaZCndomyia Zeei Stone and Barreto. (Drawn as seen through venter of cranium; from slide GML:O3848on loan courtesy of J.N. Belkin) Fig. 13. Deinocerites cancer Theobald. Fig. 14. Aedeomyia (Aedeomyial squam

Fig. 38. Sabethes (SabethCnus) undo~us (Goquillett). Fig. 39. Toxorhynchites (Toxorhynchitesl brevipaZpis Theobald. (Redrawn from Harbach and Knight 1977b) Fig, 40. Toxorhynchites Q..phCeZZa) rutilus (Coquillett) . (Anteroventral aspect) Fig. 41. Zucorethra undemoodi Underwood.

ABBREVIATIONSUSED IN FIGURES

C@ = cibarial bar LPTl = lateral premental teeth $

Dm = dorsomentum LPT2 lateral premental teeth 2

HY = hypopharynx LPT3 lateral premental teeth 3 apodeme HYB = hypopharyngeal bar PAP premental *chordate process HYM F hypopharyngeal mala PCP premental

L = lingua PCU premental cusps

LbHy = labiohypopharynx PDA premental , dental arch

Lg = ligula PLT prementoligular teeth

LP = labial palpus PM premental mala

LPP - lateral premental processes PRT premental ridge teeth

LPSl = labial palpal sensorium 1 PS premental sensorium

LPS2 = labial palpal sensorium 2 Sl superlingua

LPS3 = labial palpal sensorium 3 SM salivary meatus

LPS4 = labial palpal sensorium 4 Vm ventromentum

LPS5 F labial palpal sensorium 5 VPP ventral premental processes

LPT = lateral premental teeth 308

Vm

Fig. 1 Aedes (Ochlerotatus)

Fig. 2 Anopheles (Anopheles) Mosquito Systematics Vol. lO(3) 1978 309

------HYM

ps______

Fig. 3

Nothodixa I VbP

------HYM

pcu----__

LPT-----mm ------__ Lps

PLT- _ - _ _ _ ------_ _Lpp ___----- Fig. 4

Paradixa VbP iPSz e. CA,, d 310

LP

Fig. 5 Bifonella hfgelfa)

LPS4 LPS5 Mosquito Systematics Vol. lO(3) 1978 311

LPS3

LPT-----,_

Fig. 8

Uranofaenia (Uranotaenia) 312

DC /

Fig. 9 Uranotaenia (Pseudoficalbia)

A”‘-- HY ------,

\.‘ ip VPP LPS4

'i l ’ /

\ \

\\ ,‘-j -. _:

Fig. 10 Culex (Lutzia) Mosquito Systematics Vol. lO(3) 1978 313

\ \ I

\ .‘ _ .__ / .- ____- _ Fig. 11 Culex (Culex)

LPSy - LPS-J P PDA- LP sq

Fig. 12 Galindomyia

-._-,____-_\ ~. _ ’ LPT- ’

, ’ - -..‘ / --A V(PP ,‘ -1. - 1,” -----____. / Fig. 13 314

Fig. 14 Aedeomyia (Aedeomyia)

.I *-

,~ -‘ 1‘ ----J L‘ > ’ Fig. 15 Hodgesia Mosquito Systematics Vol. lO(3) 1978 315

\ ~~__ ’ . ------._ __ _ _

Fig. 16 Culiseta (Cullseta)’

\-:-;;-s-%X‘

Fig. 17 Mimomyia ktorleptiomyial 316

Fig. 18 Mansonia

__._----.--__C__L ___

-_

L__ / ’ l-l_ Fig. 19 Coquilletfidia koquillettidia) c. Mosquito Systematics vol. lO(3) 1978 317

HY .‘ ..

1

Fig. 20 Orthopodomyia

Fig. 21 Opifex 318

Fig. 22 Eretmapodites (Eretmapodites)

iP VPP

-1 ---____ ------_. ._.

Fig. 23 Zeugnomyia c. ef -7 Mosquito Systematics Vol. lO(3) 1978 319

Fig. 24 Udaya

Fig. 25 Armigeres (Armigeres) 320

HyB

. ’ I-.‘

Fig. 26 Heizmannia

Fig. 27 Aedes (Ochlerotatd Mosquito Systematics Vol. lO(3) 1978 321

--- __ _ _ - ---._.-

Fig. 28 Haemagagus (Haemagogd

Fig. 29 Psorophora (Janfhinosoma) 322

VPP

\___.----I------_ Fig.32 Tripferoides (Tripteroides) c. e&r I d Mosquito Systematics vol..10(3) 1978 323

../_.H-- . - ---.i..,

/------__ --I------I.

Fig. 33 Maofigoeldia I

Fig. 34 Phoniomyia _~-----._. ,-“- s‘ --. _ d .L.. _ _-c, ----_

i /i Mosquito Systematics Vol. lO(3) 1978 325

Fig. 37 Limatus

( ‘

6I Pip

VP’P

::

Fig. 38 Sabefbes (Sabefhinud 326

Fig. 39 Toxorhynchites (Toxorhynchites)

Fig. 40 Toxorhynchites (Lynchiella)

Fig. 41 Eucorefhra Mosquito Systematics vol. lO(3) 1978 327

DISCUSSION

Although capable of being retracted, the larval labiohypopharynx is only a passive participant in the feeding process. In non-predatory species the organ is said to serve as an "anvil" upon which food matter is comminuted by the mandibular teeth, In predatory species the labiohypopharynx is evidently not acted upon by the mandibles and plays an unknown role during feeding.

An examination of the "anvil" in non-predatory species reveals the presence of two basic structural forms. The first occurs in the Dixinae and Anophelinae (surface feeders); the second in the Culicinae (primarily suspension and sedi- ment feeders). In the former, the dixine hypopharyngeal malae and the anophe- line premental malae are the primary areas of the labiohypopharynx used for the breakdown of food, These areas occupy the dorsal portion of the "anvil" and occlude with the numerous peglike accessory teeth (molar areas) borne on the mesodorsal margins of the mandibles (Schremmer 1950; Fglix 1962). In the culi- tines, the ventrally-located ligula and premental teeth are the principal struc- tures used for the comminution of large food particles. These typically occlude with the ventral teeth (incisors) of the mandibles (Gardner and Nielsen 1973).

Forms intermediate between the above types are virtually unknown. Besides dixine and anopheline larvae, mandibular accessory teeth are presently known to occur only in Upanotaenia barnes; (pers. ohs.), Up

The premental teeth and the ventral mandibular teeth which occlude with them in most culicine larvae are poorly developed in surface feeders. Since as a general rule the mandibles of nematocerous larvae have strongly developed incisors and weakly developed or no molar regions, I am convinced that the re- duced incisors and the expanded molar areas of dixines and anophelines are spe- cializations associated with the surface-feeding habit. In culicines the main ventral tooth of each mandible (ventral tooth 0 of Gardner et al. 1973 and Harbach and Knight 1977a) normally occludes with the ligula (Gardner and Nielsen 1973) or the prementoligular teeth0 This tooth and the ligula are both well developed in anophelines indicating that the anopheline ligula is a plesiomorphic struc- ture and that the weakly developed incisors represent a derived condition.

The numerous scalelike ventral premental processes which occur principally in the Dixinae, Aedini and Culisetini appear to be more primitive than the small- er collection of variously modified but homologous structures found in other tribes of the Culicinae. Although the premental cordate process appears to be unique to anopheline larvae, a corresponding structure may actually occur in certain culicines. It seems to be homologous with the similarly located 'pre- mental spine" of Aedes vexans (Pao and Knight 1970) and its possible homology with the bilobed ventral premental processes of members of the tribes Uranotae- niini, Aedeomyiini, Hodgesiini and Mansoniini was noted previously. It is . possible therefore that the premental cordate process is a primitive structure which has been retained in certain taxa while the ventral premental processes have been reduced or lost. 328

I envision an ancestral culicid larva with (1) a somewhat hypognathously- carried prognathous head with mandibles which occluded with the prelabium and (2) a body similar to that of living dixine larvae but with prolegs and/or am- bulacral combs on all abdominal segments. From this hypothetical larva, I sur- mise that the first steps toward the evolution of modern culicid larvae includ- ed (1) the lateral movement of the ambulacral combs of abdominal segement IX to give rise to the pectens (this may have accompanied the formation of the spiracular apparatus) and (2) the differentiation of mandibular and premental dentitions similar to those of Qifer: but with better developed molar and malar areas, respectively. Since the spiracular lobes of extant dixine and Chagasia larvae bear a fringe of filaments, and since vestiges of these are present on the posterior spiracular lo es of various chaoborine, toxorhynchitine, culicine P and other anopheline larvae , it is likely that they were a characteristic of the prototypal culicid larva. Likewise, since many dixine and nagasia larvae rest with the body bent into a U and culicines assume the same attitude when they occasionally feed on the surface film (Horsfall 1955), the prototypal lar- va probably had the ability to assume this posture as well. The larva was pro- bably an unspecialized feeder which obtained fragments of food from the surfaces of submerged objects, flocculent masses and the surface film. It can be assum- ed that food matter was reduced to fine particles between the mandibular molar areas and the premental malae before entering the pharynx.

I believe that an ancestral stock with the above characteristics gave rise to two lines, one being the progenitor of extant dixines and the other the stem group from which the other subfamilies arose. Along the dixine line the body retained many ancestral characters while the head acquired a complete prognathous condition and the mandibles and labiohypopharynx derived the specializations necessary for feeding entirely (?) on the surface film. I suspect that the dixine hypopharyngeal malae are strictly homologous with the premental malae of Opifex and have migrated to their present location in response to a shorten- ing of the mandibles. Since the Dixinae acquired the surface-feeding habit early, enough time has elapsed for the malae to have moved to the hypopharynx and for the ligula, premental teeth and mandibular incisors to atrophy. Sur- face feeding as a primary means of obtaining food may have evolved in response to a lack of suitable nutriment as is indicated by the culicine habit of feeding at the surface under starvation conditions (Christophers 1960).

I conjecture that the aforesaid stem group (1) retained the @if=-like mandibles and labiohypopharynx; (2)2developed the ability to extract suspended particulate matter out of the water ; (3) lost all abdominal prolegs and ambu- lacral combs except those of segment VIII which moved laterally to form the 1 The vestiges- occur at the outer margins_ of the lobes and are visible in the scanning electron microscope as corrugated or undualted membranes. Montschadsky (1930) figured these as striae in species of Anopheles, Aedes, Uramotaertia, CZ&XC and CuZiseta. In addition to these genera, I have observed the structures in BironeZZa, Amigeres and Toxorhynchites larvae. The margin of each lobe is set with a row of minute filaments in Eucorethra. 2 Although living Anophelinae feed substantially on the surface film, they also remove particulate matter from suspension (Horsfall 1955). This and the recent discovery of a unique suspension-feeding chaoborine larva, AustraZomoehZonyx nitidus Freeman (Colless 1977), leads me to believe that the ability to feed on suspended matter was established along this line. Mosquito Systematics Vol. lO(3) 1978 329 combs; and (4) underwent a shortening of the abdomen and a fusion of the thora- tic segments. I feel that this ancient stock gave rise to the Anophelinae and a siphon-bearing ancestor. Once formed, the siphon-bearing progenitor diverged along two lines, one comprising the subfamily Culicinae and the other the sub- family Chaoborinae,

As with the Bixinae, the Anophelinae may have become specialized for feed- ing largely on the surface film under stress conditions, perhaps coincident with the disappearance of large swampy areas. In addition to well developed mandi- bular and premental tritural surfaces, anopheline larvae developed a number of other specializations associated with surface feeding. These include the uni- que structures which hold the feeding larva to the water's surface, i.e., the scalelike structures on the apexes of the maxillary palpi and antennae, Nuttall and Shipley's organs on the prothorax and palmate setae on abdominal segments. The combs of abdominal segment VIII have been retained in the first stage larva only.

The evolution of a siphon in the culicine-chaoborine ancestgr probably accompanied the consummation of suspension and sediment feeding. Concomitant with these developments, (1) the premental malae and mandibular molar areas atrophied and were eventually lost in most taxa; and (2) the elements of the mandibular brushes acquired rows of villiform processes for removing food par- ticles from the mouthbrushes (Colless 1977; Harbach 1977). The abdominal pec- tens were incorporated into the siphon while the combs remained on lateral areas of segment VIII.

The subfamily Chaoborinae was probably established early as a group diver- gent from the Culicinae. With the one exception noted above, all Chaoborinae larvae are predatory and have lost many of the ancestral characters. These in- clude the structures associated with suspension feeding, i.e., the mouthbrushes, combs, pectens and even the siphon in larvae of the genus Chaoborus. Conse- quently, the labiohypopharynx has been highly modified and/or reduced while the mandibles and the mandibular incisors have become strongly developed for catch- ing and holding prey.

The subfamily Toxorhynchitinae appears to have been derived from a culi- tine ancestor, specifically a sabethine, for they share some characters with this tribe. Affinities between these groups are evident in the structure of the labiohypopharynx. Both have an aggregate of prementoligular teeth and simi- lar complexes of lateral premental teeth. It should be noted that the Toxorhyn- chitinae are rapacious and like the Chaoborinae have lost the combs and pectens. Although reduced in number, the mouthbrush elements have been strengthened to help secure prey (Surtees 1959).

Taxonomy

The structural characteristics of the labiohypopharynx appear to be use- ful for separating genera but probably offer little assistance at the specific level except for the genera Urazotaenia and Psorophora. The three species ex- amined from each of these genera are quite distinct and easily separated from 3 Sediment feeding is a specialized form of suspension feeding where settled particles are stirred up by the action of the mouthbrushes and then collected from the water. 330 one another. The labiohypopharynges of larvae representing such genera as Ano- phe les, Mansonia, F?yeomy%aand especially Toxorhyndzities are very difficult if not impossible to distinguish even when examined with the scanning electron mi- croscope. The morphology of the labiohypopharynx might be used for the recog- nition of the subgenera of A&es, and perhaps those of other aedine genera.

ACKNOWLEDGMENTS

I am indebted to Mrs. Chien C. Chang who skillfully prepared all of the drawings and to the following individuals who generously provided larval spe- cimens either directly to me or to Kenneth L. Knight: Richard G. Andre, U.S. Army Medical Component - AFRIMS, Bangkok, Thailand; John N. Belkin, University of California at Los Angeles; Mary E. Fisher, University of Illinois at Urbana- Champaign; James S. Haeger, Florida Entomological Research Laboratory, Vero Beach; Charles L. Hogue, Los Angeles County Museum; Peter F. Mattingly, The British Museum of Natural History; George F. O'Meara, Florida Medical Entomo- logy Laboratory, Vero Beach; Shivaji Ramalingam, University of Malaya, Kuala Lumpur; Philippe Rossignol, Department of Microbiology and Parasitology, Uni- versity of Toronto, Ontario; Alan Stone (retired); Robert C. Wallis, Yale Uni- versity School of Medicine, New Haven, Connecticut; and Ronald A. Ward, Medical Entomology Project, Smithsonian Institution. I am grateful to Dr. John N. Bel- kin, University of California at Los Angeles, Dr. Graham B. White, The Bri- tish Museum of Natural History and Dr. Kenneth L. Knight, North Carolina State University, Raleigh for critically reviewing the manuscript. Mosquito Systematics vol. lO(3) 1978 331

APPENDIX

The species examined in addition to those illustrated are listed below by subfamily and in alphabetical order.

Anophelinae

AnopheZes (AnopheZesl cruetins Wiedemann

Chaoborinae .

Corethre 2Za Zaneana Vargas

Culicinae

Aedeomyia (Aedeomyial catas ticta Knab Aedes (OchZerotatus) taeniorhynchus (Wiedemann) Aedes (Mkidus) painei Knight CuZex (Lutxia) bigot5 Bellardi CuZex (MeZanoconion) opisthopus Komp CuZex (Meknoeonion) pizosus (Dyar and Knab) CuZiseta k%macura) meZanura (Coquillett) Deinocerites pseudes Dyar and Knab Haemagogus(Haemagogus) panarchys Dyar MaZaya sp . Mansonia (Mansonio%des)uniformis (Theobald) Mtiomyia (Mimomyia) &an&erZaini Ludlow OrthopodomyiaphyZlozoa (Dyar and Knab) Phoniomyia spZendida (Bonne-Wepster and Bonne) Psorophora (Grabhamia) pygmaea (Theobald) Psorophora (Psorophora) ciZiata (Fabricius) Sabethes (Sabethes) eyaneus (Fabricius) Ttichoprosopon (Trichoprosopon) digitatum (Rondani) Uranotaenia (Uranotaenia) geometrka Theobald Wyeomyia (Wyeomyia) grayii Theobald

Dixinae

Dixa sp. 332

LITERATURE CITED

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